The present invention relates to a backup technique for a storage device that is equipped with a disk driving device for storing data received from an information processing apparatus and a cache memory for temporarily retaining data to be stored in the disk driving device.
A disk array device in which an uninterruptible power source having a minimum necessary rated output power is used as a means for reliably backing up a cache memory having a large storage capacity is known. In this disk array device, a write cache save area that is separate from a user data area is formed in each of a plurality of HDDs (hard disk drives) that constitute an arbitrary array among a plurality of arrays that are managed by a disk array controller having the cache memory. When a power failure has occurred, the contents of the cache memory are collectively written to the write cache save areas of the HDDs (JP-A-2000-357059).
Incidentally, to respond, at high speed, to access from a host computer (hereinafter referred to as “host”) as a host apparatus of a hard disk storage device (hereinafter abbreviated as “storage device”), the hard disk storage device is equipped with a cache memory such as a DRAM which is volatile. With this configuration, data to be transferred from the host to the storage device are written to and retained temporarily by the cache memory before being written to the HDD. At this instant the storage device informs the host about the completion of data writing, whereby high-speed response to access from the host is secured.
On the other hand, with the recent trends of downsizing, open architecture, etc. in the IT environment, storage devices having the above kind of configuration have come to be used more frequently in environments where power failures occur at a high frequency. Therefore, it is important to take a proper measure to secure data retained by a volatile cache memory at the occurrence of a power failure. The following two methods are commonly employed as such a measure.
In the first method, when a power failure has occurred, the storage device is driven intermittently by supplying high power from a backup power source to the storage device in as short a time as several minutes and data retained by the cache memory are transferred to and written to the HDD. However, in this method, there may occur a case that data retained by the cache memory cannot be written to the HDD completely, because the processing of transferring the data inside the storage device and a circuit configuration relating to the writing of the data to the HDD are complex and the execution of the above kinds of processing requires a large number of devices. That is, there is a risk of losing part of data retained by the cache memory.
In the second method, when a power failure has occurred, low power is supplied from a backup power source to only the cache memory for as relatively long a time as several days to back up only the cache memory. This method has an advantage of high reliability because only a small number of devices need to be driven. However, the period during which data retained by the cache memory are secured, that is, the backup period, is limited because a battery module as the backup power source can supply power only in a period determined by its capacity.
As described above, it is difficult for either of the two methods to completely secure data retained by the cache memory. On the other hand, employing both methods increases the size of the battery module that is incorporated in the storage device. As a result, not only the size of the storage device itself but also the device cost is increased.